Development of a Thermal Plasma Moment Analyzer Based on a Superconducting Line (2-D) Dipole Electromagnet

a. Research Objectives

One of the most difficult experimental problems in space physics is making detailed measurements of the properties of the cold thermal plasma (ionized gas) in the ionosphere and plasmasphere. Contemporary models of the thermodynamics and transport properties of the ionosphere and electrodynamics of the aurora have developed a considerable degree of detail in their treatment of the velocity space distribution function of the thermal plasma. In order to validate these models and make further progress, the scientific requirement on the experiments is to be able to specify the thermal plasma velocity distribution function in sufficient detail to calculate the lowest order twenty velocity moments, including electric current density, heat flow vector, and the off-diagonal terms of the pressure tensor. In this proposal, we will request support for the design and prototype testing of a cold plasma analyzer that will be capable of measuring the thermal plasma velocity distribution function with much greater detail than is presently available. In this context, we will use the term cold to mean plasmas with temperature T 10 eV with particular emphasis on temperature T 1 eV.

There are many reasons why these measurements are very difficult to make. First, an in situ spacecraft lacks a true ground. The operating point of plasma measuring instruments on a spacecraft is often determined by reference to telemetry or buss ground. Depending on what instruments are operating, the amount of insolation and other factors, this potential may vary by ~10kT/e or more with respect to ambient plasma potential. In these circumstances, cold plasma particles may be repelled or accelerated in the plasma sheath around the spacecraft. Techniques exist that will mitigate and correct for these problems. However, these techniques introduce uncertainties that make it difficult to measure higher order moments of the thermal plasma velocity distribution function. Second, cold plasma particles lack sufficient energy to exceed the noise or window thresholds of many detectors based on particle counting techniques that are commonly used to study the properties of more energetic space plasmas. Third, the density of most cold plasmas is large enough to overwhelm or saturate particle counting techniques with high counting rates. Spaceborne in situ measurement of cold ionospheric thermal plasmas is usually accomplished by means of Langmuir probes, Faraday cups or rf probes. These instruments have very coarse angular resolution. They are only capable of measuring density, temperature, temperature and/or pressure anisotropy, and bulk flow velocity. This level of capability is inadequate to meet the needs of contemporary models or to solve a number of vexing observational controversies.

This proposal requests support for the task of developing a preliminary idea of how to solve this experimental problem into a sufficiently detailed and tested instrument design to enable us to prepare and submit proposals to NASA for in situ testing in space.

a.1: Relationship to Previous Work:

The idea is one that was suggested by Prof. T. S. Huang, a theorist at the Physics Department of Prairie View A &M University, based on a paper of his that recently appeared [Huang and Birmingham, 1994]. The proposed work will be carried out in conjunction with Dr. Huang, who will carry out a detailed theoretical analysis and simulation of charged particle trajectories in the various alternative configurations that will emerge during the design process.

The Principle Investigator's disseratation work was based on Langmuir probe studies of high moments of the plasma distribution function. He has more than 25 years experience with probe and particle detector studies of thermal plasmas in the ionsphere, plasma distribution functions and plasma waves.

a.2: Scientific Need:

There are several scientific questions that can be addressed by the instrument that we are proposing to develop. These questions include the identification of the component of the plasma that carries the downward Birkeland currents near auroral arcs. Historically, it has been assumed that this current is carried by an upward bulk drift of the entire thermal electron population in the ionospheric plasma. However, evidence has accumulated to indicate that this current may instead be carried by a magnetically aligned beam of suprathermal ionospheric electrons [Collin et al., 1982; Klumpar et al., 1982]. Verification of this conclusion and elucidation of the driving mechanism responsible for producing such beams are important unsolved questions in auroral physics. A second major area of investigation concerns the development of strongly non-Maxwellian plasma distribution functions in the ionosphere whenever the electric field exceeds 70 mV/m [Schunk and Walker, 1971; Schunk, 1975; Schunk et al., 1975, 1994; St.-Maurice and Schunk, 1979]. This model prediction has been supported by evidence inferred from high latitude incoherent scatter radar data, but has never been directly verified [Schunk et al., 1994]. Finally, the growth and propagation of wave modes in the ionospheric plasma depends entirely on the details of the plasma velocity space distribution function. The absence of data on the plasma distribution function at thermal energies has required data analysts and modelers alike to use approximations and assumptions that limit our abilities to predict instabilities and plasma wave spectra [Bering et al., 1987].

Technologically, it is important and interesting to monitor the plasma velocity space distribution function in several applications. These include any space-based industrial processes that occur in vacuum in low earth orbit, many industrial plasma processes and controlled thermonuclear fusion reactors. The proposed instrument will have useful applications in these areas.

a.3: Relation to Other NASA Programs:

The scientific questions that might be answered through spaceborne use of the proposed instrument are directly relevant to the scientific objectives of programs such as TIMED, UARS, and FAST. Once developed, we plan to propose test flights on (probably) both a sounding rocket and a SPARTAN package. Further use would depend on wehat projects or opportunities were announced in the early 2000's time frame.

b. Research personnel

b.1: At the University of Houston:

The personnel required to accomplish the research at UH are the Principal Investigator, the Co-Investigator one graduate student, two undergraduates and two staff engineers. The principal investigator is a faculty member in the Physics Department of the University of Houston. The senior investigator have more than fifty years experience in the study of upper atmospheric physics and space and atmospheric electricity using ground instruments, balloons and sounding rockets. The staff engineers are N. K. Mahale, an electromagnet designer with the Texas Center for Superconductivity at the University of Houston (TCSUH), and Johan Flick of the Physics Department. The design of the instrumentation will be done by the staff engineers with assitance from the graduate student under direction of the senior investigators. The instrumentation electronics will be fabricated in part by the students in the electronics shop of the Space Physics Group at the University of Houston under the direction of the Co-Investigator. Analysis of the prototype test data will be done by the graduate students under the direction and guidance of the principal investigator.

b.2: At Prairie View A&M University:

The personnel required at PVAMU are the Co-PI, a programmer, a graduate student and an undergraduate. The Co-PI and the graduate student develop a detailed theoretical treatment of particle motion in the proposed instrument. This model will be implemented in numerical form by the programmer. The theoretical work at PVAMU will provide guidance to the mechanical design team.

c. Methodology

c.1: Experimental:

The proposed instrument exploits the unique kinetic and thermodynamic properties of a convecting plasma in a 2-dimensional dipole magnetic field [Huang and Birmingham, 1994]. In a recent paper, Huang and Birmingham have showed that the guiding center motion of charged particles is considerably simpler if the magnetic field is that of a 2-D rather than a 3-D dipole. The former is also known as a line dipole field. It is produced by pair of closely spaced wires carrying oppositely directed currents equal in magnitude and infinitely extended in what we define to be the y-direction. In the jargon of high energy particle and accelerator physics, this type of magnet is called a racetrack magnet. A magnetic field of the order of 1 T is required to keep the gyroradious of thermal ions small compared to the dimensions of the detector surface. For initial strawman purposes, we will assume superconducting cables separated by 2 cm, carrying 4.0 x 105 A, with long axis in the y direction, as shown in Figure 1. Currents of this magnitude would involve prohibitive power dissipation for spacecraft use if ordinary conductors were used, so we have based our design on a superconducting magnet.

Magnetic field lines lie entirely in the transverse x-z plane and field intensity B drops off with cylindrical radius = (x2+z2)1/2 as -2. Plasma will be admitted to the region of this field through a narrow slit lying in the x-y plane with its long axis parallel to B located at z=+~40 cm. An electric field of ~3000 V/m will be applied in the -y-direction. A position sensitive detector (see below) will be located at z=+6cm in the x-y plane, as shown in Figures 2 and 3.

In the detection region, the magnetic field intensity will be such that the gyroradius will be 1 cm for 10 eV protons and much less for electrons. Thus, the guiding center approximation can be used to treat particle motion within the detector. In the guiding center approximation, for the line dipole, equatorial pitch angle e and mirror colatitude angle m are equal (taking the acute value of each). The drift velocity is given by the three usual contributions, the E x B, the magnetic gradient and the magnetic curvature components. For the proposed configuration, the E x B drift will convect plasma from the entrance slit to the detector surface. The guiding centers will drift in such a way that their equatorial pitch angles and, therefore, mirror ``latitudes'', remain constant! The x coordinate of position on the detector surface is proportional to the tangent of the``latitude'' angle. Thus, there will be a unique and well understood correspondence between x coordinate on the position sensitive detector and initial pitch angle of the particle. As they drift inwards, the kinetic energy of the particles will increase as z-2, where z is the midplane crossing distance of the magnetic field line threading the guiding center.

In the absence of space charge, the guiding center trajectory of a particle within the detector will be given by

qE(y-y0)+mv20 z20/z2=const.

where subscript ``0" means the initial value. Thus, the y value of detector intercept is a linear function of initial particle energy. In other words, the image that appears on the detector is an image in energy-pitch angle coordinates of the thermal plasma distribution function, a quantity no other instrument has ever been able to measure. In fact, two images would be obtained, one of ions and the other of electrons. Depending on ambient density and available telemetry, it should be possible to obtain statistically significant images in 100 ms.

The fact that an appreciable amount of thermal plasma will be flowing into the interior of the instrument means that the applied electric field will be opposed by a polarization field owing to the separation of charges within the previously neutral plasma. The nominal applied field within the instrument , 3000 V/m, is greater than the largest polarization field of ~1000 V/m that could be produced by an ambient 1012 m-3 plasma, assuming a 1 cm wide entrance aperture, allowing for abiabatic compression within the instrument. The effect of polarization will be to reduce the convection velocity within the instrument and to change the y dependence of the instrument response. Modeling the effect of polarization will be one of the year 1 design tasks.

We have already conducted an extensive review of possible position sensitive detectors for use in this configuration. The relatively low energy of the incoming particles (~1-50 eV) and the relatively high currents (which may reach 10 nA in the central pixels) make most of the detectors used in high energy physics inappropriate for this application. The detectors that appear most likely to work are small planar Langmuir probes operating at fixed potential withlinear current to voltage converter preamplifiers (Figure 4) situated on the detector plane immediately behind the detector surfaces. Preliminary discussions with the staff of Texas Components, Inc. of Houston, Texas indicates that it should be possible to assemble hybrid preamps in a small enough package to fit an 8 x 16 array into a 20 cm x 40 cm area. This detector array will output a set of 128 analog voltages that are proportional to the currents being collected by the patch probes. These voltages will be sampled and digitized and read out to the telemetry bus of whatever vehicle the instrument is being flown on. A block diagram of the instrument electronics is shown in Figure 4.

The cooling scheme envisioned for the proposed prototype would be utilize exteranllly supplied cryogenics. This approach is adfequate for sounding rocket and short orbital missions. The experience of NASA with spaceborne infrared observatories has shown that wth suitable insulation schemes, it is possible to maintain liquid helium temperatures on orbit for intervals of at least a year.

The foregoing conceptual sketch is most of the design of this instrument concept that exists at the present time. Concepts that need to be fleshed out in considerable substance include details of the magnet, many of the details of the detection scheme, a workable design for external magnetic shielding, details of the electrostatics of the entrance aperture and the method to be used for maintaining the entrance aperture guard ring at plasma potential. This proposal requests support for this design work, which will take approximately one year to complete, including time for running simulation codes on the entrance aperture optics, and for a prototype construction and preliminary testing, which will occupy years 2 and 3 of the project. We will construct the prototype to ``sounding rocket" specifications, so that it can be further tested via either a dedicated sounding rocket or a SPARTAN flight. Prototype construction will also provide good estimates of power, weight and volume requirements, parameters which will be needed to write an actual spaceflight proposal.

c.2: Theoretical:

There are two major design problems that will require extensive modelling by the theory team to resolve. The first question concerns the efect of space charge on the particle energy and y-intercept on the detector surface as a function of ambient plasma density, since the electrostatically induced polarization field will reduce the effective E felt locally by individual particles. This problem will be treated through numerical treatments of a coupled set of Possion's Equation and a guiding center model of plasma motion. The other problem concerns the design of electrostatic inlet ``optics" that minimizes perturbations of the ambient plasma distribution. The theoretical treatment will consist primarly of detailed particle trajectory tracing simulations of various inlet aperture and instrument interior configurations. In addition, the effect of the real non-ideal 2D dipole field on the particle guiding center trajectory will be evaluated.

d. Technology transfer

Our plan for technology transfer is primarily an incremental one that focusses on some of the specific parts required to build the overall instrument. The main thrust of the effort will be a development program conducted in concert with the research staff at Texas Components,Inc. aimed at advancing state-of-the-art in the area of arrays of hybrid, high impedance current to voltage converters. Further commercial development of the completed instrument will depend on the outcome of the prototype tests and will probably not occur until after the completion of the International Space Station.

e. Institutional commitment and sources of additional support

e.1: Additional support:

The University of Houston has already provided modest start-up support for the task of converting this very preliminary instrument idea into a preliminary design that has sufficient detail for inclusion in this proposal. This support has come from the Institute for Space Systems Operations (2 weeks of PI salary and 6 months of graduate student support) and TCSUH (1 month of PI salary, a CAD workstation and design supplies).

e.2: Institutional commitment at the University of Houston:

The University of Houston Physics Department has three faculty members working in the area of Upper Atmospheric and Space Physics. The Space Physics group occupies 3800 square feet of laboratory and office space. Of particular importance is the complete balloon and rocket instrument fabrication facility operated by the Space Physics Group. The University has also provided the Space Physics Group with two computers for data analysis. The main Physic Department computer is a DEC AlphaServer 2000/233 with 64 MB RAM, 18 BG of disk, of which 8.6 GB is reserved for Space Physics, a TZ87 20GB cartridge tape drive, a CD-ROM reader, and network connectiosn to a variety of other peripherals. The SPace Science Data Center facilities include the University of Houston Space Physics Computer, which is a dedicated VAX11/750 with 16 MB of main memory and 1.6GB of disk space, two tape drives and a variety of other peripherals. In addition, the group has a total of 8 PC workstations all connected to the Campus LAN.

e.2: Institutional commitment at Prairie View A&M University:

The Prairie View A&M University Physics Department has been designated by the NASA Historically Black Colleges and Universities (HBCU) program as one to be developed into a center of excellence in space science. Recently, NASA has committed support for the construction of a Laboratory for Space Radiation at PVAMU. The Co-PI is a memeber of this laboratory, where he conducts studies of the space environment. In addition, NASA has supported two research projects of the Co-PI, including the purchase of computer facilites. The Department of Physics is committed to building a space physics group with the objective of training minority students in space science.

f. Impact on infrastructure of science and engineering

The primary infrastructure impact of this research will be on the education of scientists at the University of Houston and Prairie View A&M University. Experimental space science programs are, in general, excellent preparation for students interested in research into space and atmospheric science. Students will acquire practical, hands-on experience in the process of developing scientific instruments and models, instrumentation development, instrument testing operations, data system design, etc. The students will receive one of the most intensive and rewarding learning experiences available in space science. It is widely recognized that participation in the space program constitutes one of the broadest possible interdisciplinary technical education opportunities. We also expect that the cross disciplinary skills of two UH staff engineers will be improved. Finally, the staff of Texas Components, Inc have indicated that the process of meeting our pre-amplifier requirements will add at least one new product to their company's product line, and will certainly contribute to the development of their skill and expertise in supplying spacecraft components.

g. Summary

g.1: Work Plan:

In year 1 we will develop a detailed design for the magnet, refine the plasma ``optics" of the entrance aperture and develop a quantitative model of the role of space charge. We will develop a detailed cryostat design. We will intitatie electronic design. We will initiate magnet procurement. In year 2, we will complete mechanical and electronic designs, procure the detector array and complete fabricaiton of the prototype. In year 3, we will conduct lab plasma tests of the instrument, in test plasmas on campus and at other local plasma chambers, if available.

g.2: Scientific Closure:

In an instrument development project, consideration of scientific closure has two aspects. First, within the narrow confines of the project itself, the major question is does the instrument work as intended. Second, the larger issue of how the developed instrument might be used in future projects to answer the questions posed above.

The answers to the first question will be provided in several stages. The instrument electronics and magnet will be extensively bench tested in the lab using a current source instead of a plasma to stimulate the patch probe array. Second, the response to low currents of ions and electrons will be tested in a vacuum chamber using collimated low energy electron and ion sources available here on campus. This stage of testing will provide preliminary pitch angle calibration. Third, the entire instrument will be tested in a plasma chamber using the low energy ionospheric simulation plasma source that the UH College of Technology has developed.

The answers to the second set of questions will depend, of course, on our success in building and testing the instrument. Once we have succeeded, it is our intention to propose several different flight programs for this instrument into the ionosphere on both auroral zone sounding rockets and low earth orbit, high inclination satellites. One might, for example, envision flying this instrument on a mother-daughter sounding rocket through an active auroral arc, with thermal plasma analyzer deployed on a daughter to help minimize magnetic cleanliness probems. The main payload might comprise a reasonably comprehensive array of auroral particle and field detectors. Closure on questions such as the occurence of non-Maxwellian distribtuion functions could be acheived by measuring the ambient electric field with a double probe and the plasma distribution function with the proposed instrument and comparing the predictions of the models with observation. Closure on questions regarding the generation of plasma waves by such non-Maxwellian distribution functions could be answered by oberving the wave spectra on the mother payload and carrying out computations similar to those reported by Bering et al. [1987]. The observed pitch angle distribution of thermal electrons would contribute to understanding the Birkeland current question posed above.

h. Bibliography

Bering, E. A., J. E. Maggs, and H. R. Anderson, The plasma wave environment of an auroral arc, 3. VLF Hiss, J. Geophys. Res., 92, 7581-7605, 1987.

Collin, H. L., R. D. Sharp, and E. G. Shelley, The occurrence and characteristics of electron beams over the polar regions, J. Geophys. Res., 87, 7504, 1982.

Huang, T. S., and T. J. Birmingham, Kinetic and thermodynamic properties of a convecting plasma in a two-dimensional dipole field, J. Geophys. Res., 99, 17295-17308, 1994.

Klumpar, D. M., and W. J. Heikkila, Electrons in the ionospheric source cone: Evidence for runaway electrons as carriers of downward Birkeland currents, Geophys. Res. Lett., 9, 873-876, 1982.

St.-Maurice, J.-P., and R. W. Schunk, Ion velocity distributions in the high latitude ionosphere, Rev. Geophys., 17, 99-134, 1979.

Schunk, R. W., Transport equations for aeronomy, Planet. Space Sci., 23, 437, 1975.

Schunk, R. W., and J. C. G. Walker, Transport processes in the E region of the ionosphere, J. Geophys. Res., 76, 6159, 1971.

Schunk, R. W., W. J. Raitt, and P. M. Banks, Effect of electric fields on the daytime high-latitude E and F regions, J. Geophys. Res., 80, 3121, 1975.

Schunk, R. W., L. Zhu, and J. J. Sojka, Ionospheric response to traveling convection twin vortices, Geophys. Res. Lett., 21\/}, 1759- 1762, 1994.

j. Figures

Figure 1. A plan view of the proposed detector, showing the y-z plane in the midplane of the instrument.

Figure 2. An end-on view of the proposed instrument, showing the x-z plane near y=0.

Figure 3. A 3-d rendering of one end of the instrument, with the electrode and suppressor grid removed.

Figure 4. A block diagram of the electronics.